Conventional biomedical sensors, including immunoassays based systems, have been used to report the presence and/or concentration of a wide variety of analytes. Immunoassays are generally classified into two categories: competition assays and sandwich assays. In a competition assay, the antigen in the test sample is mixed with an antigen-probe complex (commonly referred to as a reporter complex) and the mixture then competes for binding to the antibody. The probe may be a radioisotope, a fluorophore, or a chromophore. In a sandwich immunoassay, the antigen in the test sample binds to the antibody and then a second antibody-probe complex binds to the antigen. In these prior art assay methods, one or more washing steps are usually required. The washing steps introduce complexity into the assay procedure and can generate biohazardous liquid waste.
Immunoassays usually provide a user with either a qualitative result (e.g., a “yes/no answer”) obtained, most often by a simple visual detection (e.g., color change), or a quantitative result such as a concentration of an antigen. Most of the quantitative methods involve expensive pieces of equipment, such as scintillation counters (for monitoring radioactivity), spectrophotometers, spectrofluorimeters (see, e.g., U.S. Pat. No. 5,156,972), surface plasmon resonance instruments (see, e.g., U.S. Pat. No. 5,965,456), and the like. It would therefore be advantageous to develop an immunoassay that is both inexpensive and simple enough to use to be suitable for home or field use. Such an biosensor would preferably require no centrifugation, dilution, pipetting, washing, or timing steps, and would generate minimal waste.